A production method for carbon nanotubes each with a diameter of 0.4 nanometers to several tens of nanometers is known, this method being based on a vapor phase growth method. In the present specification and the like, examples of carbon nanofibers include fishbone graphitic-structure nanotubes each of which is hollow, that is, tubular, but includes a graphitic layer that is not parallel to a fiber axis, ribbon-like graphitic fibers that are not hollow, and graphitic whiskers.
Carbon nanotubes or carbon nanofibers produced by the vapor phase growth method need to be recovered by being separated from gas. Japanese Patent Laid-Open No. 2006-169002 discloses a technique to catch carbon nanotubes or nanofibers carried by a carrier gas, on a filter such as a net or a plate with holes. When the carrier gas passes through the net or the like, the carbon nanotubes or nanofibers are caught on the net or the like.
When an obstacle such as a filter is provided in the carrier gas flow to catch the carbon nanotubes or nanofibers, an increase in the amount of carbon nanotubes or nanofibers increases the channel resistance to the carrier gas flow and thus the pressure on the upstream side of the filter. The elevated pressure may increase the pressure in a reactor in which the carbon nanotubes or nanofibers are synthesized, or disturb the gas flow in the reactor. This may affect the production process.